Remove unused projects pagination code

[MyPyDavid]

Removes rdmo/projects/templates/projects/projects_pagination.html and the set_context_querystring_with_filter_and_page util function.

Otherwise we could have used the new {% querystring %} templatetag from https://docs.djangoproject.com/en/5.2/releases/5.1/#what-s-new-in-django-5-1 for the pagination.

[coveralls]

Coverage is 95.156% — 2.5.2/fix/clean-up-projects-pagination-template into 2.5.2/release. No base build found for 2.5.2/release.

[MyPyDavid]

we can merge it now in 2.5.2 (it only deletes files/code).
However it will also be relevant for the big rebase and cleanup for 3.0.0 https://github.com/rdmorganiser/rdmo/milestone/25, eg. in #1582 .